Methods and apparatus for measurement of relative critical dimensions

ABSTRACT

One embodiment relates to a method of measuring a relative critical dimension (RCD) during electron beam inspection of a target substrate. A reference image is obtained. A region of interest is defined in the reference image. A target image is obtained using an electron beam imaging apparatus. The target and reference images are aligned, and the region of interest is located in the target image. Measurement is then made of the RCD within the region of interest in the target image. Another embodiment relates to a method of measuring a RCD which involves scanning along a scan length that is perpendicular to the RCD. Point RCDs along the scan length are measured. A filter is applied to the point RCDs, and an average of the point RCDs is computed. Other embodiments, aspects and features are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims the benefit of U.S. provisional patentapplication No. 61/731,580, entitled “Relative Critical DimensionMeasurement for Hotspot Inspection and Control,” filed Nov. 30, 2012 byinventor Hong Xiao, the disclosure of which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to inspection and review of substrates,such as, for example, semiconductor wafers and reticles.

2. Description of the Background Art

In a conventional charged-particle beam instrument, such as an electronbeam (e-beam) inspection instrument, a manufactured substrate (such as asilicon wafer or a reticle) is scanned with a focused beam of electronswhich results in the emission of secondary electrons from the substratesurface. The emitted electrons are detected, and the detection data istypically converted into images of the surface of the specimen. Theseimages are then analyzed numerically to detect abnormalities (referredto as defects) in the manufactured substrate.

SUMMARY

One embodiment relates to a method of measuring a relative criticaldimension (RCD) during electron beam inspection of a target substrate. Areference image is obtained. A region of interest is defined in thereference image. A target image is obtained using an electron beamimaging apparatus. The target and reference images are aligned, and theregion of interest is located in the target image. Measurement is thenmade of the RCD within the region of interest in the target image.

Another embodiment relates to a method of measuring a RCD which involvesscanning along a scan length that is perpendicular to the RCD. PointRCDs along the scan length are measured. A filter is applied to thepoint RCDs, and an average of the point RCDs is computed.

Another embodiment relates to an apparatus including a source forgenerating an incident electron beam, a scanning system for controllablydeflecting the incident electron beam to scan the incident electron beamover a surface such that secondary electrons are emitted therefrom, anda detection system for detecting the secondary electrons so as togenerate an image data. The apparatus further includes a control andprocessing system that is programmed to obtain a reference image, definea region of interest in the reference image, obtain a target image,align the target and reference images, locate the region of interest inthe target image, and measure a RCD within the region of interest in thetarget image.

Other embodiments, aspects and features are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an charged-particle beam apparatus inaccordance with an embodiment of the invention.

FIG. 2 is a flow diagram of a method for the measurement of a relativecritical dimension in conjunction with hot spot defect inspection inaccordance with an embodiment of the invention.

FIG. 3 depicts an illustrative example of a reference image from areference site inspection in accordance with an embodiment of theinvention.

FIG. 4 depicts an illustrative example of a region of interest (ROI)within a reference image inspection in accordance with an embodiment ofthe invention.

FIG. 5 depicts an illustrative example of a target image of aninspection site inspection in accordance with an embodiment of theinvention.

FIG. 6 depicts an illustrative example showing target and referenceimages that are aligned to each other inspection in accordance with anembodiment of the invention.

FIG. 7 depicts an illustrative example showing an ROI located in thetarget image in accordance with an embodiment of the invention.

FIG. 8 depicts an illustrative example of a gap feature within an ROIlocated in the target image in accordance with an embodiment of theinvention.

FIG. 9 is a flow diagram of a method of measuring a relative criticaldimension within an ROI in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

Methods and apparatus for the measurement of relative criticaldimensions using a charged-particle beam apparatus are disclosed herein.These methods and apparatus may be employed advantageously inconjunction with “hot spot” defect inspection.

FIG. 1 provides a cross-sectional diagram of the charged-particle beamimaging apparatus 100 based on a scanning electron microscope (SEM)which may be utilized in accordance with an embodiment of the invention.As shown in FIG. 1, a source 101 generates an incident electron beam(primary electron beam) 102. The incident charged particle beam,electron beam 102, passes through a Wien filter 104. The Wien filter 104is an optical element configured to generate electrical and magneticfields which cross each other. Scanning deflectors 106 and focusingelectron lenses 107 are utilized. The scanning deflectors 106 areutilized to scan the charged-particle beam across the surface of thetarget substrate 110. The target substrate 110 may be, for example, apatterned substrate, such as an integrated circuit being manufactured ora reticle for lithography.

The focusing electron lenses 107 are utilized to focus the incidentelectron beam 102 into a beam spot on the surface of the wafer or othersubstrate sample 110. In accordance with one embodiment, the focusinglenses 107 may operate by generating electric and/or magnetic fields.

As a result of the scanning of the incident electron beam 102, secondaryelectrons are emitted or scattered from the surface of the targetsubstrate 110 (which may be, for example, a semiconductor wafer or areticle). The target substrate 110 may be held by a movable stage 111.The secondary electrons are then extracted from the target substrate 110by exposure to the electromagnetic field of the objective (final) lens108. The electromagnetic field acts to confine the emitted electrons towithin a relatively small distance from the incident electron beam opticaxis and to accelerate these electrons up into the column. In this way,a secondary electron beam 112 is formed from the secondary electrons.

The Wien filter 104 deflects the secondary electrons 112 from the opticaxis of the incident electron beam 102 to a detection axis (the opticaxis for the detection system 114 of the apparatus). This serves toseparate the secondary electrons 112 from the incident electron beam102. The detection system 114 detects the secondary electrons 112 andgenerates data signals that may be utilized to create images of thesurface of the target substrate.

An instrument control and data processing (control/processing) system150 may include one or more processors (i.e. microprocessors ormicrocontrollers) 152, data storage (including, for example, hard diskdrive storage and memory chips) 154, a user interface 156 and a displaysystem 158. The data storage 154 may be configured to store or holdcomputer-readable program code (instructions) 155 and data, and theprocessor 152 may be configured to execute the program code 155 andprocess the data. The user interface 156 may be configured to receiveuser inputs. The display system 158 may be configured to display viewsof the substrate surface to a user.

The control/processing system 150 may be connected to, and may be usedto control, various components of the charged-particle beam column so asto implement procedures disclosed herein. For example, the movement ofthe stage 111, and the scanning by the deflectors 106 may be controlledby computer-readable program code 155 executed by the control/processingsystem 150.

In addition, the control/processing system 150 may be configured toreceive and process the electron image data from the detection system114. In particular, the computer-readable program code 155 in thecontrol/processing system 150 may be used to implement proceduresrelating to the relative critical dimension measurement technique whichis described herein.

Furthermore, in accordance with an embodiment of the invention, thecontrol/processing system 150 may be configured to interface with a dataserver 170. The data server 170 may be configured to store designpattern data 172 and extracted image data 174. During an inspection run,the data server 170 may provide said data in real time at the request ofthe control/processing system 150 of the charged-particle beam imagingapparatus 100.

FIG. 2 is a flow diagram of a method 200 for the measurement of arelative critical dimension in conjunction with defect inspection inaccordance with an embodiment of the invention. The method 200 may beperformed using the charged-particle beam imaging apparatus 100, forexample.

Per step 202, site location information (i.e. the locations of the sitesto be inspected) may be loaded by the data processing system from asource. The site location information may be, for example, “hots spots”of a design rule check.

Alternatively, the site location information may comprise defectlocations in inspection results from a previous inspection by thecharged-particle beam imaging apparatus 100 or by another inspectionapparatus.

For each site location, a procedure 210 for hot spot inspection and RCDmeasurement may be performed. As shown, the procedure 210 may involveperforming the following steps.

Per step 212, a reference image may be loaded into a data processingsystem, such as the control/processing system 150, for example. In oneembodiment, the reference image may be a SEM image that is acquired froma reference site by an imaging apparatus, such as the charged-particlebeam imaging apparatus 100, for example. The reference site ispreferably a known good (defect free) site that corresponds to aninspection site. In another embodiment, the reference image may berendered from design data which may be retrieved by the charged-particlebeam imaging apparatus 100 from the data server 170, for example.

An illustrative example of a reference image from a reference site isshown in FIG. 3. As depicted, the reference image 300 may includemultiple features (302, 304, 306, 308, 310, 312, 314, 316, 318, and320). While illustrative example shows only the outline of each feature,each feature may be distinguishable in the reference image by contrastagainst the background or surrounding pixels. For example, each featuremay have darker pixels against a background of lighter pixels of thereference image. Alternatively, each feature may have lighter pixelsagainst a background of darker pixels. For SEM images, lighter pixelstypically correspond to a greater intensity of secondary electronsdetected, and darker pixels typically correspond to a lower intensity ofsecondary electrons detected.

Per step 214, a region of interest (ROI) within the reference image maybe defined. An illustrative example of a ROI 400 within a referenceimage is shown in FIG. 4. As depicted, the ROI 400 may be a rectangularshape. In this particular example, the ROI 400 includes portions of twofeatures (304 and 306) that are adjacent or nearby each other.

Per step 216, a target image of the inspection site may be collected oracquired. The target image may be an SEM image and may be collectedusing the charged-particle beam imaging apparatus 100, for example.

An illustrative example of a target image 500 is shown in FIG. 5. Thistarget image 500 corresponds to the reference image 300 depicted in FIG.3. As depicted, the target image 400 may include multiple features (502,504, 506, 508, 510, 512, 514, 516, 518, and 520) which correspond to(but may vary somewhat from) the multiple features (302, 304, 306, 308,310, 312, 314, 316, 318, and 320) in the reference image 300. Again,while illustrative example shows only the outline of each feature, eachfeature may be distinguishable in the reference image by contrastagainst the background or surrounding pixels.

Per step 218, the target image and the reference image may be aligned toeach other. The alignment process may involve, for example, shifting thepositioning of the target image relative to the reference image untilthere is a good match between them. The goodness of the match may bedetermined, for example, by a least squares fit. Other alignmentprocesses may be used in other implementations.

An illustrative example showing target and reference images that arealigned to each other is depicted in FIG. 6. In FIG. 6, the outlines ofthe features (502 through 520) of the target image are shown in solidlines, while the outlines of the matching features (302 through 320,respectively) from the reference image are shown in dashed lines. Asshown, the target and reference features correspond to each other, butthey may differ in their exact shapes and positioning.

Per step 220, the ROI 700 may be located in the target image 500, asdepicted in FIG. 7. In this particular example, the ROI 700 includesportions of the two target features (504 and 506) that correspond to thetwo reference features (304 and 306, respectively) in the correspondingROI 400 located in the reference image 300.

Per step 222, an inspection or review procedure may be performed todetermine whether the ROI has a defect and/or to classify the defect.This procedure may be performed using existing techniques.

Per step 224, a relative critical dimension (RCD) measurement may beperformed. The RCD measurement may be performed to determine a RCD for aspecified feature within the ROI. The RCD measurement per step 224 maybe performed in parallel with the defect detection and/or classificationper step 222.

In an illustrative example, the specified feature may be the width ofthe gap feature between edges of the two line features (504 and 506) inthe ROI 700. This gap feature is depicted in FIG. 8. In this example,there are two gap portions (802 and 804) in between which the gapappears to be closed. In another example, the specified feature may be aline, rather than a gap.

The RCD of the specified feature (for example, the width of a gap or aline) may be measured relative to a pixel size. In other words, the RCDof the specified feature may be determined in terms of a number ofpixels. The number of pixels may be converted to a distance (forexample, in nanometers) based on the magnification of the image. Such afeature width which is based on the image pixels may be referred to asan RCD measurement because it is a measurement relative to the imagepixels rather than an absolute measurement in nanometers.

In accordance with an embodiment of the invention, the RCD may bemeasured in accordance with the exemplary method 900 shown in FIG. 9.Alternatively, other similar methods may be utilized.

In the exemplary method 900, the specified feature may be scanned alonga length that is perpendicular to the critical dimension (the lengthbeing referred to herein as the “scan length” or the “perpendicularlength”). The scanning may be done increments of one or more pixels. Forexample, the length perpendicular to the RCD within the ROI may be 1024pixels long and may be numbered 1 through 1024. In this case, scanningin increments of one pixel would result in 1024 scan points, andscanning in increments of two pixels would result in 512 scan points,and so on. An increment for the scanning may be determined or selectedper step 901. It is anticipated that a scan increment of one pixel maybe typically selected.

Per step 902, a next scan point along the perpendicular length may beselected. The RCD at the scan point (the “point RCD”) may then bemeasured per step 904. The point RCD may be measured in terms of anumber of pixels. Example point RCDs 1002 for the example gap featurebetween two lines (504 and 506) are illustrated by the arrows in FIG.10.

In the example shown in FIG. 10, each point RCD may be measured byconsidering the vertical column of pixels at the horizontal scan point.A derivative may be performed on the pixel intensities in the column ofpixels so as to determine a function indicative of the change of pixelintensity along the column. Positive and negative thresholds may then beapplied to the change of pixel intensity to select the start and endpixels of the specified feature at that point. The point RCD may then bedetermined as a number of pixels from the start pixel to the end pixel.

The method 900 may continue to select a next scan point along theperpendicular length and measure the point RCD at the scan point, untilit is determined per step 906 that there are no further scan points tobe selected. In accordance with an embodiment of the invention, once allthe point RCDs have been measured, then optional filtering may beperformed on the point RCD data per step 908.

In one embodiment, the filter applied may be a nearest-neighbor (NN)filter. The NN filter may average each point RCD with itsnearest-neighbor point RCDs. In other words, the NN filtered point RCDfor scan point j may be the average of the three raw point RCDs for scanpoints j−1, j and j+1. Alternatively, the NN filter may weight the rawpoint RCD for the middle point j by a weighting factor. For example, ifthe weighting factor is two, then the NN filtered point RCD for scanpoint j may effectively average the four raw point RCDs for scan pointsj−1, j, j and j+1.

In another embodiment, the filter applied may be a next-nearest-neighbor(NNN) filter. A NNN filter may average each point RCD with itsnearest-neighbor and next-nearest-neighbor point RCDs. In other words,the NNN filtered point RCD for scan point j may be the average of thefive raw point RCDs for scan points j−2, j−1, j, j+1 and j+2.Alternatively, the NNN filter may weight the raw point RCD for themiddle point j by a weighting factor. For example, if the weightingfactor is two, then the NNN filtered point RCD for scan point j mayeffectively average the six raw point RCDs for scan points j−2, j−1, j,j, j+1 and j+2.

In another embodiment, the filter applied may be anext-next-nearest-neighbor (NNNN) filter. A NNNN filter may average eachpoint RCD with its nearest-neighbor, next-nearest-neighbor, andnext-next-nearest-neighbor point RCDs. In other words, the NNNN filteredpoint RCD for middle point j may be the average of the seven raw pointRCDs for scan points j−3, j−2, j−1, j, j+1, j+2 and j+3. Alternatively,the NNNN filter may weight the raw point RCD for scan point j by aweighting factor. For example, if the weighting factor is two, then theNNNN filtered point RCD for scan point j may effectively average theeight raw point RCDs for scan points j−3, j−2, j−1, j, j, j+1, j+2 andj+3.

The boundary values may be dealt with in various ways. Consider thatthere are M scan points such that j ranges from 1 to M. One way to dealwith boundary values is described below, but other implementations arealso possible.

In one implementation, non-existent nearest neighbor scan points may beignored in computing the averages. For example, the NN filtered pointRCD (without weighting of the middle point) may be computed at scanpoint 1 to be the average of the two point RCDs for scan point 1 and 2and may be computed at scan point M to be the average of the two pointRCDs for scan points M−1 and M. As another example, the NN filteredpoint RCD with double weighting of the middle point may be computed atscan point 1 to effectively average the three point RCDs for scan point1, 1 and 2 and may be computed at scan point M to effectively averagethe three point RCDs for scan points M−1, M and M.

Returning to FIG. 9, after the optional filtering per step 908, variousRCD characteristics may be computed per step 910. In one embodiment, theRCD characteristics include an average RCD, a maximum RCD, a minimumRCD, and a standard deviation for the RCD, a short/open flag, and ashort/open length.

Each RCD characteristic may be computed based on the raw and/or filteredpoint RCD data. The average RCD may be computed by averaging all the rawpoint RCDs, or alternatively all the filtered point RCDs. The minimumRCD may be determined by finding the shortest raw point RCD, oralternatively finding the shortest filtered point RCD. The maximum RCDmay be determined by finding the longest raw point RCD, or alternativelyfinding the longest filtered point RCD. The standard deviation may becomputed as the standard deviation of the raw point RCDs, oralternatively as the standard deviation of the filtered point RCDs.

The short/open flag may be used to indicate whether there is a closedgap indicative of a short circuit between two lines or a broken lineindicative of an open circuit. The short/open flag may be set if theminimum RCD is determined to be zero. The short/open length may beprovided if the short/open flag is set. The short/open length may becomputed, for example, based on the number of pixels along the length ofthe gap or line having zero for the point RCD (either raw or filtered).In other words, the run length of the short/open may be computed andoutput.

In the above description, numerous specific details are given to providea thorough understanding of embodiments of the invention. However, theabove description of illustrated embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. One skilled in the relevant art will recognize that theinvention can be practiced without one or more of the specific details,or with other methods, components, etc. In other instances, well-knownstructures or operations are not shown or described in detail to avoidobscuring aspects of the invention. While specific embodiments of, andexamples for, the invention are described herein for illustrativepurposes, various equivalent modifications are possible within the scopeof the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. A method of measuring a relative criticaldimension during electron beam inspection of a target substrate, themethod comprising: obtaining a reference image; defining a region ofinterest in the reference image; obtaining a target image using anelectron beam imaging apparatus; aligning the target and referenceimages; locating the region of interest in the target image; andmeasuring a relative critical dimension (RCD) within the region ofinterest in the target image, wherein measuring the RCD comprisesscanning along a scan length that is perpendicular to the RCD, andmeasuring point RCDs along the scan length, wherein measuring a pointRCD comprises performing a derivative on pixel intensities in a verticalcolumn of pixels at a horizontal scan point to determine a functionindicative of a change in pixel intensity along the vertical column ofpixels, and applying positive and negative thresholds to the change inthe pixel intensity.
 2. The method of claim 1, wherein measuring the RCDfurther comprises: averaging the point RCDs.
 3. The method of claim 2further comprising: applying a filter to the point RCDs.
 4. The methodof claim 3, wherein the filter comprises a nearest-neighbor type filterthat averages the point RCD with neighboring point RCDs.
 5. The methodof claim 4, wherein the nearest-neighbor type filter averages the pointRCD with a preceding point RCD and a subsequent point RCD along the scanlength.
 6. The method of claim 4, wherein the nearest-neighbor typefilter averages the point RCD with two preceding point RCDs and twosubsequent point RCDs along the scan length.
 7. The method of claim 4,wherein the nearest-neighbor type filter averages the point RCD withthree preceding point RCDs and three subsequent point RCDs along thescan length.
 8. The method of claim 4, wherein the nearest-neighbor typefilter weights the point RCD.
 9. The method of claim 2 furthercomprising: determining if the point RCDs include a zero value to detecta short/open characteristic.
 10. The method of claim 9 furthercomprising: computing a run length of the short/open characteristic. 11.The method of claim 2 further comprising: computing a minimum RCD, amaximum RCD and a standard deviation for the RCD.
 12. An apparatuscomprising: a source for generating an incident electron beam; ascanning system for controllably deflecting the incident electron beamto scan the incident electron beam over a surface such that secondaryelectrons are emitted therefrom; a detection system for detecting thesecondary electrons so as to generate an image data; and a control andprocessing system programmed to obtain a reference image, define aregion of interest in the reference image, obtain a target image, alignthe target and reference images, locate the region of interest in thetarget image, and measure a relative critical dimension (RCD) within theregion of interest in the target image, wherein the RCD is measured byscanning along a scan length that is perpendicular to the RCD, andmeasuring point RCDs along the scan length, wherein a point RCD ismeasured by performing a derivative on pixel intensities in a verticalcolumn of pixels at a horizontal scan point to determine a functionindicative of a change in pixel intensity along the vertical column ofpixels, and applying positive and negative thresholds to the change inthe pixel intensity.
 13. The apparatus of claim 12, wherein the controland processing system is further programmed to measure the RCD byaveraging the point RCDs along the scan length.
 14. The apparatus ofclaim 13, wherein the control and processing system is furtherprogrammed to apply a filter to the point RCDs.
 15. The apparatus ofclaim 14, wherein the filter comprises a nearest-neighbor type filterthat averages the point RCD with neighboring point RCDs.
 16. Theapparatus of claim 15, wherein the nearest-neighbor type filter weightsthe point RCD.
 17. The apparatus of claim 13, wherein the control andprocessing system is further programmed to determine if the point RCDsinclude a zero value to detect a short/open characteristic.
 18. Theapparatus of claim 17, wherein the control and processing system isfurther programmed to compute a run length of the short/opencharacteristic.
 19. The apparatus of claim 13, wherein the control andprocessing system is further programmed to compute a minimum RCD, amaximum RCD and a standard deviation for the RCD.
 20. A method ofmeasuring a relative critical dimension (RCD), the method comprising:scanning along a scan length that is perpendicular to the RCD; measuringpoint RCDs along the scan length, wherein measuring a point RCDcomprises performing a derivative on pixel intensities in a verticalcolumn of pixels at a horizontal scan point to determine a functionindicative of a change in pixel intensity along the vertical column ofpixels, and applying thresholds to the change in the pixel intensity;applying a filter to the point RCDs; and averaging the point RCDs. 21.The method of claim 20, wherein the filter comprises a nearest-neighbortype filter that averages the point RCD with neighboring point RCDs.